Some vehicles are provided with an occupant protection system for protecting an occupant at a time of vehicle collision. For example, an occupant protection system deploys an airbag at collision to protect an occupant. Recently, there is a demand that an occupant protection system protects an occupant against not only a primary collision of a vehicle but also a secondary collision. The secondary collision is the collision with another object after the primary collision (first collision) and the irregular movement of the vehicle resulting from the impact of the primary collision.
JP-2005-280380A discloses a conventional occupant protection system. In the system, acceleration of a side part of a vehicle after the rotation of the vehicle due to the collision (primary collision) is detected with a yaw rate sensor, so that the side collision occurring as the secondary collision is detected based on the detected yaw rate.
The inventor of the present application believes that in an occupant protection system, it is required to further reduce damage of an occupant suffered at a time of the secondary collision. In particular, in the secondary collision, a vehicle side collision may occur during rotation of the vehicle and the occupant may be unstable in the rotating vehicle. In this case, the occupant moves in conjunction with the rotation of the vehicle and easily collides with an inner surface of a vehicle cabin. The collision with the inner surface of the vehicle cabin may result in a large damage of the occupant.